In many circumstances, a cost-effective means of recovering natural gas from a reservoir is to produce the natural gas with consequent decline in reservoir pressure until an economic lower limit of productivity is reached. Frequently, when pressure in the natural gas reservoir decreases to a sufficiently low level, compression is instituted to improve productivity. At the low pressures often associated with the conclusion of such depletion operations, the molar quantity of natural gas still remaining in the reservoir is small and secondary recovery techniques for this residual quantity are not normally cost effective. In some reservoirs, natural gas zones are associated with underlying zones containing heavy oils. There are special difficulties associated with recovering heavy oils, and in some circumstances the depletion of gas zone overlying a heavy oil zone can interfere with subsequent efforts to recover the heavy oil.
A variety of processes are used to recover heavy oils and bitumen. Thermal techniques may be used to heat the reservoir to produce the heated, mobilised hydrocarbons from wells. One such technique for utilising a single horizontal well for injecting heated fluids and producing hydrocarbons is described in U.S. Pat. No. 4,116,275, which also describes some of the problems associated with the production of mobilised viscous hydrocarbons from horizontal wells.
One thermal method of recovering viscous hydrocarbons using two vertically spaced horizontal wells is known as steam-assisted gravity drainage (SAGD). Various embodiments of the SAGD process are described in Canadian Patent No. 1,304,287 and corresponding U.S. Pat. No. 4,344,485. In the SAGD process, steam is pumped through an upper, horizontal, injection well into a viscous hydrocarbon reservoir while hydrocarbons are produced from a lower, parallel, horizontal, production well vertically spaced proximate to the injection well. The injection and production wells are typically located close to the bottom of the hydrocarbon deposit.
It is believed that the SAGD process works as follows. The injected steam initially mobilises the in-place hydrocarbon to create a “steam chamber” in the reservoir around and above the horizontal injection well. The term “steam chamber” means the volume of the reservoir which is saturated with injected steam and from which mobilised oil has at least partially drained. As the steam chamber expands upwardly and laterally from the injection well, viscous hydrocarbons in the reservoir are heated and mobilised, especially at the margins of the steam chamber where the steam condenses and heats a layer of viscous hydrocarbons by thermal conduction. The mobilised hydrocarbons (and aqueous condensate) drain under the effects of gravity towards the bottom of the steam chamber, where the production well is located. The mobilised hydrocarbons are collected and produced from the production well. The rate of steam injection and the rate of hydrocarbon production may be modulated to control the growth of the steam chamber to ensure that the production well remains located at the bottom of the steam chamber in an appropriate position to collect mobilised hydrocarbons.
Alternative primary recovery processes may be used that employ thermal and non-thermal components to mobilise oil. For example, light hydrocarbons may be used to mobilise heavy oil. U.S. Pat. No. 5,407,009 teaches an exemplary technique of injecting a hydrocarbon solvent vapour, such as ethane, propane or butane, to mobilise hydrocarbons in the reservoir.
In the context of the present application, various terms are used in accordance with what is understood to be the ordinary meaning of those terms. For example, “petroleum” is a naturally occurring mixture consisting predominantly of hydrocarbons in the gaseous, liquid or solid phase. In the context of the present application, the words “petroleum” and “hydrocarbon” are used to refer to mixtures of widely varying composition. The production of petroleum from a reservoir necessarily involves the production of hydrocarbons, but is not limited to hydrocarbon production. Similarly, processes that produce hydrocarbons from a well will generally also produce petroleum fluids that are not hydrocarbons. In accordance with this usage, a process for producing petroleum or hydrocarbons is not necessarily a process that produces exclusively petroleum or hydrocarbons, respectively. “Fluids”, such as petroleum fluids, include both liquids and gases. Natural gas is the portion of petroleum that exists either in the gaseous phase or is in solution in crude oil in natural underground reservoirs, and which is gaseous at atmospheric conditions of pressure and temperature. Natural Gas may include amounts of non-hydrocarbons.
It is common practice to segregate petroleum substances of high viscosity and density into two categories, “heavy oil” and “bitumen”. For example, some sources define “heavy oil” as a petroleum that has a mass density of greater than about 900 kg/m3. Bitumen is sometimes described as that portion of petroleum that exists in the semi-solid or solid phase in natural deposits, with a mass density greater than about 1000 kg/m3 and a viscosity greater than 10,000 centipoise (cP; or 10 Pa.s) measured at original temperature in the deposit and atmospheric pressure, on a gas-free basis. Although these terms are in common use, references to heavy oil and bitumen represent categories of convenience, and there is a continuum of properties between heavy oil and bitumen. Accordingly, references to heavy oil and/or bitumen herein include the continuum of such substances, and do not imply the existence of some fixed and universally recognized boundary between the two substances. In particular, the term “heavy oil” includes within its scope all “bitumen” including hydrocarbons that are present in semi-solid or solid form.
A reservoir is a subsurface formation containing one or more natural accumulations of moveable petroleum, which are generally confined by relatively impermeable rock. An “oil sand” or “tar sand” reservoir is generally comprised of strata of sand or sandstone containing petroleum. A “zone” in a reservoir is merely an arbitrarily defined volume of the reservoir, typically characterised by some distinctive property. Zones may exist in a reservoir within or across strata, and may extend into adjoining strata. In some cases, reservoirs containing zones having a preponderance of heavy oil are associated with zones containing a preponderance of natural gas. This “associated gas” is gas that is in pressure communication with the heavy oil within the reservoir, either directly or indirectly, for example through a connecting water zone.
A “chamber” within a reservoir or formation is a region that is in fluid communication with a particular well or wells, such as an injection or production well. For example, in a SAGD process, a steam chamber is the region of the reservoir in fluid communication with a steam injection well, which is also the region that is subject to depletion, primarily by gravity drainage, into a production well.